There is a strong association between exposure of the skin to the ultraviolet light component of sunlight and the development of skin cancers, such as malignant melanoma and the non-melanoma skin cancers, mainly basal cell carcinomas (BCCs) and squamous cell carcinomas (SCCs). The incidence of these cancers has been rapidly increasing world wide. In Britain, there were 4000 newly-diagnosed cases of malignant melanoma in 1994, an 80% increase over the past 10 years (Wessex Cancer Trust, 1996). In the United States, approximately 34,100 new cases were expected, an increase of 4% per year. Queensland, Australia, has the highest incidence of melanoma in the world, but early detection and widespread public health campaigns and the promotion of the use of sunscreens and reduction of ultraviolet exposure have helped to reduce the number of deaths. BCCs currently affect one in 1,000 in the U.K. population, and the incidence has more than doubled in the last 20 years (Imperial Cancer Research Fund, U.K., 1997). One million new cases of BCCs and SCCs are expected to be diagnosed in the USA in 1997, compared to 600,000 in 1990 and 400,000 in 1980 (National Oceanic and Atmospheric Administration U.S.A., 1997). In Australia, there is no reason to suspect that a similarly increasing incidence would not also apply, despite extensive publicising of the dangers of solar and UV radiation, with the Queensland population being at the greatest risk.
Over 90% of all skin cancers occur on areas of the skin that have been regularly exposed to sunlight or other ultraviolet radiation, with U.V.B. responsible for burning the skin and associated with malignant melanomas, and U.V.A. associated with premature skin aging and the development of BCCs and SCCs (Wessex Cancer Trust, 1996). Childhood sun exposure has been linked to the development of malignant melanoma in younger adults. Other risk factors include a genetic predisposition (fair complexion, many skin moles), chemical pollution, over-exposure to X-rays, and exposure to some drugs and pesticides. Depletion of the ozone layer of the stratosphere is considered to contribute to long-term increases in skin cancer.
Surgical removal is by far the most common treatment for malignant melanomas, BCCs and SCCs. This can take the form of electrodesiccation and curettage, cryosurgery, simple wide excision, micrographic surgery or laser therapy. Other treatments, used when the cancers are detected at a later stage of development, are external radiation therapy, chemotherapy or to a lesser extent bio-immunotherapy or photodynamic therapy. The choice of treatment is dependent on the type and stage of the disease and the age and health of the patient (National Cancer Institute, U.S.A., 1997).
All of the present treatments suffer from severe limitations. The major concern is the poor recognition of cancerous cells at the site of excision and the high likelihood of recurrence, necessitating follow-up surgery and treatment, with the risk of further disfigurement and scarring. In one publication, the reported rates for incompletely-excised BCCs was 30-67% (Sussman and Liggins, 1996). Immune suppression associated with surgery may cause any remaining cells to proliferate, and increase the risk of metastases. In melanoma patients there is a high risk that the cancer has already metastasized at the time of initial surgery, and late recurrence leading to death is common. Present alternatives to surgery, such as radiation therapy and chemotherapy, also carry risks of immune suppression and poor specificity. Immunotherapy and gene therapy hold the greatest promise, but the rational application of these is likely to be still decades away.
When the tumour is past the stage amenable to surgery, the most common treatment for melanoma or metastatic skin cancer of all types is chemotherapy, which has been largely unsuccessful (Beljanski and Crochet, 1996)
In theory, an ideal drug would be one that when applied topically to an exposed melanoma, BCC or SCC, selectively necrotises the tumour cells or induces them to undergo apoptosis, without causing damage to the surrounding healthy skin cells. In practice, this has yet to be achieved. The drugs currently available are neither selective nor penetrative.
The lay public is also enamoured of the concept of topical chemotherapy. There have been many documented “home remedies” for skin cancer, which have had disastrous consequences, eg the use of boot polish (Adele Green, Queensland Institute of Medical Research, pers. Comm.) The major danger is the production of scar tissue, underneath which the tumour cells continue to grow. An extract derived from plants of the genus Solanum (kangaroo apple or devil's apple) and purportedly containing solasodine glycosides has been available in Australia as a non-prescription preparation treatment of sunspots and solar keratoses, under the name “Curaderm”. However the preparation was shown in a small clinical trial against BCCs to be ineffective, with 14/20 patients showing persisting tumour on histological examination of tissue from the treated site. In some cases, histological examination of the site of treatment revealed malignant tissue embedded in scar tissue. The authors warned against self-diagnosis and treatment, particularly with irritant substances (Francis et al, 1989).
However, anecdotal reports suggest that plant sap extracts are still being used by the general public for the treatment of sunspots or solar keratoses, with some success being claimed.
The sap of plants of the family Euphorbiaceae, particularly the genus Euphorbia, has been used in the folk medicine of many countries. The genus was named after an early Greek physician in deference to its purported medicinal properties (Pearn, 1987). Only recently have some of these claims been investigated scientifically. The genus is enormously diverse, ranging from small, low-growing herbaceous plants to shrubs and trees. Nearly all reports of activity of these plants and their extracts are anecdotal or derived from traditional medicine, and the nature of the preparations used is frequently either unknown or very poorly described. Activity has been claimed against a huge variety of conditions, ranging from warts, “excrescences”, calluses, “cheloid tumours”, corns, whitlows or felons, “superfluous flesh” and the like, to a variety of cancers (see, for example, Hartwell: Lloydia 1969 32 153).
As part of the screening program for anti-cancer activity carried out on 114,000 extracts from 35,000 terrestrial plant species carried out by the United States National Cancer Institute, a number of species of Euphorbia were tested. An aqueous suspension, an olive-oil suspension, an alcohol extract and an acid extract were screened for activity against the transplantable tumour cell line sarcoma 37. Four species were tested. Of these, Euphorbia peplus showed no activity in any of the extracts; Euphorbia drummondii, Euphorbia pilulifera, and Euphorbia resinifera showed weak activity of an acid extract, an alcohol extract, and an olive-oil suspension respectively (Belkin and Fitzgerald, 1953). A review of the scientific and medical literature of the past five years revealed a diversity of powerful active principles such as di- and tetra-terpenes, flavonoids, sterols and proteins in this genus, and many bioactive effects have been reported, with both positive and adverse effects noted. These reports are summarized in Table 1. In particular the genus Euphorbia is well known to produce tumour promoters such as phorbol esters (Hecker, E.: “Cocarcinogens from Euphorbiaceae and Thymeleaceae” in “Symposium on Pharmacognosy and Phytochemistry” (Wagner et al, eds., Springer Verlag 1970 147-165)).
TABLE 1SpeciesActive principleActionReferenceEuphorbiawhole plant:prostatic and lungOksuz, S.aleppicaaleppicatines,neoplasmset al (1996)diterpene polyesters,cycloartene triterpenes,scopoletin,kaempferol, 4-hydroxybenzoic acidEuphoribacerebrosides?Falsone Gbiglandulosaet al (1994)Desf.Euphorbialatexskin irritant andGundidza,bougheiitumour promotingM. et aleffect(1993)Euphorbialatex: lipasehomologyMoulin, A.characias(43.5%) with Bet al (1994)chain of ricinEuphorbiawhole plant:skin irritantGundidza,coopereiphorbol esterM. et alNE Br(1992)Euphorbiaalkaline extracttreatment ofLiu Y,fisherianaepilepsyet al (1994)Euphorbiawhole plantinhibition ofVijaya, K,hirtabacteria of Shigellaet al (1995)sppEuphorbiawhole plant:antidiarrhoeicGlavez, J.hirtaflavonoidactivityet al (1993)Euphorbiawhole plant:?Yoshida, T.humifusahydrolysableet al (1994)tannins, polyphenolglucosideEuphorbiaroot:Chinese herbalGuo, Z.hylonoma3,3′,4-tri-O-met-medicine ?? actionet al (1995)methylellagic acid,betasitosterolEuphorbiawhole plant:stimulation ofMatsumoto,kansuiingenolsexpression of theT. et almacrophage Fc(1992)receptorEuphorbiapelletised plantrodenticideGassling andlathyrismaterialLandis(1990) U.S.Pat. No.4906472Euphoribalatexmitogenic lectinStirpe, F.marginataet al (1993)Euphoriba? quercetin,Folk remedies forWeedon andpeplushyperoside,warts, corns,Chickkaempferol,asthma, rodent(1976) andsitosterol, alkaloids,ulcer, BCCreferencesglycosidescitedthereinEuphorbiaditerpenesselectively cytotoxicFatope,poisoniifor human kidneyM. O. et alcarcinoma cell(1996)line A-498Euphorbialatexinhibition ofJurberg, P.splendensmollusc(1995)Biomphalariaglabrata (vectorsof schistosomiasis)Euphorbiawhole plantreduces EBV-Imai, S.tirucallispecific cellular(1994)immunity inBurkitt's lymphoma
The most intensively studied species of this group is Euphorbia pilulifera L (synonyms E. hirta L.; E. capitata Lam.), whose common names include pill-bearing spurge, snake-weed, cat's hair, Queensland asthma weed and flowery-headed spurge. The plant is widely distributed in tropical countries, including India, and in Northern Australia, including Queensland. According to the “Encyclopedia of Common Natural Ingredients Used in Food, Drugs and Cosmetics” (Leung and Foster, 1996), the whole flowering or fruiting plant is used in herbal remedies, principally for cough preparations, and in traditional medicine for treatment of respiratory conditions such as asthma, bronchitis, coughs and hayfever. This reference reports the active constituents of Euphorbia pilulifera to be choline and shikimic acid, and that other compounds present include triterpenes, sterols, flavonoids, n-alkanes, phenolic acids, L-inositol, sugars and resins. Of these components, shikimic acid is an essential intermediate in the synthesis of aromatic amino acids, and has been reported to have carcinogenic activity in mice (Evans and Osman, 1974; Stavric and Stoltz, 1976). Jatrophanes, ingenanes, and a tetracyclic diterpene designated pepluane were identified in the sap of Euphorbia peplus by Jakupovic et al (1998a). The jatrophanes were stated to have a different conformation from those of previously-known jatrophanes. Jatrophanes are also stated to belong to a group of non-irritant diterpenes, which could have accounted to their being overlooked in previous studies. There is no disclosure or suggestion at all of any biological activity of the jatrophanes or of the new pepluane compound; nor is it suggested that any of these compounds might be useful for any pharmaceutical purpose.
A recent report describes selective cytotoxicity of a number of tigliane diterpene esters from the latex of Euphorbia poisonii, a highly-toxic plant found in Northern Nigeria, which is used as a garden pesticide and reputed to be used in homicides. One of these compounds has a selective cytotoxicity for the human kidney carcinoma cell line A-498 more than 10,000 times greater than that of adriamycin (Fatope et al, 1996).
In a series of patent applications, Tamas has claimed use of Euphorbia hirta plants and extracts thereof for a variety of purposes, including tumour therapy (EP 330094), AIDS-related complex and AIDS (HU-208790) and increasing immunity and as an antifungoid agent for treatment of open wounds (DE-4102054).
Thus, while there are isolated reports of anti-cancer activity of various Euphorbia preparations (see Fatope et al, 1996; Oksuz et al, 1996), not only are the compounds present in at least one Euphorbia species reported to be carcinogenic (Evans and Osman, 1974; Stavric and Stolz, 1976; Hecker, 1970; 1977), but at least one species has a skin-irritant and tumour-promoting effect (Gundidza et al, 1993), and another species reduces EBV-specific cellular immunity in Burkitt's lymphoma (Imai, 1994).
To our knowledge, there has been no reliable or reproducible report of the use of any extract from Euphorbia species for the treatment of malignant melanoma or SCCs. An anecdotal report of home treatment of a BCC with the latex of Euphorbia peplus (petty spurge or milk weed) was the publication of Weedon, D. and Chick, J., entitled “Home treatment of basal cell carcinoma” (1976). The authors stated that medicinal propeties have been claimed for the milky juice of this plant since the time of Galen, and it was widely used as a home remedy for corns, warts, and asthma. At the turn of the century it was used by some physicians in Sydney for the treatment of rodent ulcers. The author's patient claimed to have treated himself over many years for multiple BCCs.
“The patient, a 54 year old male, had been seen sporadically at the Royal Brisbane Hospital since 1971. On one visit he was noted to have a clinical basal cell carcinoma on the anterior part of his chest which was confirmed by biopsy of a tiny specimen taken from one edge. Some days later when the biopsy site had healed the patient applied the sap of Euphorbia peplus every day for 5 days. The area became erythematous and then pustular, after which the lesion sloughed off. On his return 6 weeks after treatment, the patient agreed to let us surgically excise the small area of residual scarring. Multiple sections showed dermal scar tissue which contained a few chronic inflammatory cells, but showed no evidence of residual tumour.”
The authors stated that “this communication should in no way be taken as a recommendation of the form of therapy”. There are a few reports cautioning on the corrosive nature of the sap, and minor eye damage that has resulted from the home treatment of warts using Euphorbia peplus (Eke, T., 1994). It appears likely that the effect reported by Weedon and Chick resulted from the irritant activity of the Euphorbia peplus sap, and that, as in the case of the Solanum extract “Curaderm” reported by Francis et al (1989), there is a high risk of residual tumour cells surviving in or under the scar tissue that results from such treatment.
The inventor has now surprisingly found that sap of plants from three different Euphorbia species, Euphorbia peplus, Euphorbia hirta and Euphorbia drummondii, specifically inhibits growth of three different human tumour cell lines, including malignant melanoma. Moreover, at very low concentrations, sap from Euphorbia peplus and Euphorbia hirta induced differentiation of malignant melanoma cells so that they adopted the morphological appearance of normal melanocytes. At similar or even lower concentrations an extract stimulated activation of the metallothionein gene promoter and expression of a reporter gene in MM96L malignant melanoma cells. The results were particularly striking, since the melanoma cell line which was used is refractory to inhibition by all of the conventional chemotherapeutic agents which have been tested against it (Maynard and Parsons, 1986).